Utility functions are only defined up to addition of a constant and scaling by a positive constant. So instead of rewarding them with +5 and punishing them with -5, you can use 1005 and 995 instead. Problem solved.
The numbers are indeed arbitrary. But ultimately you want to avoid low utility/reward action and continue high utility/reward actions. That behavior, trying to avoid or pursue actions, would be indicative of the state of distress regardless of an arbitrary number attached to it.